Yeah, be very careful with that bro. If you don't close your positions and don't instruct the broker what you wish to do (usually by 2:30), you're basically leaving it to their discretion which means you're at their mercy for sticking it to you (especially if there's afterhours or weekend news that could affect the price of the stock)! Normally for call options, they will close the position if you don't have the money and you don't have a margin account. But if they forget to do that, or you go into the money by even $0.01 (normally) right before close, they would automatically get exercised and then the broker will liquidate your position first thing on Monday morning (and who knows what the stock price will be then). Different brokers have different rules and you should check the specific ones for your brokerage, but that's generally what happens and it could end up being a very costly mistake.
This happened to me with some call options with LNKD. Even though my account was tax sheltered (meaning I could not even transfer those large sums to it), I ended up in the money by exactly a penny at 4:00 p.m., they ended up buying 1000 shares of LNKD @ over $100, even though there was hardly any money in that account and then the stock opened $3+ lower on Monday and just like that, it was a $3500 loss.